Various types of power amplifiers have been proposed. A class E power amplifier is incorporated in, for example, a power transmission apparatus in a wireless power transmission system to amplify power for transmission. In general, the class E power amplifier achieves high amplification efficiency through an operation called zero-cross switching. The zero-cross switching operation is implemented by turning on and off a switch element provided in the class E power amplifier according to a switching frequency.
When the class E power amplifier is incorporated in the power transmission apparatus, power transmission conditions are not always constant. For example, the distance between the power transmission apparatus and a power reception apparatus may vary, an obstacle may be present near the power transmission and reception apparatuses, or the load impedance of the power reception apparatus may vary. The switching frequency suitable for the zero-cross switching operation varies depending on a variation in the transmission conditions. That is, the variation in transmission conditions may disrupt the zero-cross switching operation, thus increasing a conduction loss in the switching element and reducing amplification efficiency.
To suppress the decrease in the amplification efficiency, a technique to correct the switching frequency depending on the variation in the transmission conditions is expected to be used. However, a power transmission frequency needs to be variable in order to allow such a technique to be applied. Thus, such a technique cannot be applied in a situation where the power transmission frequency is not allowed to be changed because of legal restrictions or the like. Therefore, avoiding the decrease in the amplification efficiency is difficult.